Aromatic polyimides are normally prepared from an aromatic tetracarboxylic dianhydride and an aromatic primary diamine. When these materials are reacted at relatively low temperatures in a suitable solvent, typically a dipolar aprotic solvent such as N-methylpyrrolidone or N,N-dimethylacetamide, an aromatic polyamic acid is formed, usually as a viscous solution sometimes referred to as a varnish. When heated to a temperature above about 140.degree. C. imidization occurs such that a polyimide polymer is formed.
It is known from Japan Kokai No. 57-200452 and Japan Kokai No. 57-200453 that finely divided aromatic polyimides of a variety of aromatic tetracarboxylic acids and aromatic diamines can be formed by rapidly heating to 160.degree.-300.degree. C. solutions of carboxy anhydride group-free polymers in a polar organic solvent such as N-methylpyrrolidone, N,N-dimethylformamide, etc. In this way, polyimide powders suitable for use in compression molding were formed from solutions of such copolymers as 3,3',4,4'-biphenyltetracarboxylic acid-4,4'-diaminodiphenyl ether copolymer, pyromellitic acid-4,4'-diaminodiphenyl ether copolymer, and 3,3',4,4'-biphenyltetracarboxylic acid-4,4'-diaminodiphenylmethane copolymer.
Polyimide polymers based on use of 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane as the sole or predominant aromatic primary diamine are of considerable interest because of their desirable high temperature properties. However the production of such polymers in powder form presents a number of problems. Certain prior methods for converting the polyamic acid to the corresponding polyimide yield the polyimide in the form of solids which require extensive grinding, and substantial portions of the product may not even be amenable to grinding. Other prior methods are fraught with difficulties caused by the tendency of the wet polyimide polymer to agglomerate into stringy or tacky masses which can foul reactor and agitator surfaces. Further, such tacky masses cannot be removed from the reactor in any commercially practical manner. Moreover, the solvent tends to remain occluded in such swollen, tacky masses.
A desirable contribution to the art would be a process in which such difficulties may be eliminated, or at least greatly reduced. This invention is deemed to constitute such a contribution